1. Field of the Invention
The present invention relates to an image reading apparatus and method suitable for use in the correction of skew error of a medium.
2. Description of the Related Art
In recent years, an image reading apparatus has been developed that allows documents of multiple sizes to be read. For this purpose, the image reading apparatus is provided with detection means for detecting the size of documents.
Operation of the conventional image reading apparatus will be described below with reference to a flow chart of FIGS. 4A and 4B, and an explanatory view of a sensor unit of FIG. 5. The sensors shown in FIG. 5 are sensors of a transmission type, and a skew angle is detected with sensors SB4, SB5, SF1, SA4, and SA3.
Before starting the reading of documents, an operator first sets the size of the document (S1). Then, at the start of reading, the rotation of a conveyance motor for driving a conveyance roller or the like is started (S2). When a predetermined time, for example 50 msec, has elapsed after the start of the rotation of the conveyance motor, a first clutch and a second clutch are switched on to start driving the conveyance roller (S4).
When the first document is picked, it is determined whether or not any of a plurality of size sensors (4 sensors in the example shown) has been turned, that is, whether a document has been detected or not (S5). If it is determined at step S5 that no document has been detected by the size sensors, it is determined whether SF1 has detected a document or not(S6). If, at step S6, it is determined that no document has been detected by SF1, it is determined at step S7 whether a certain time (2 seconds in this example) has elapsed after the start of picking of documents or not. If, at step S7, it is determined that the 2 seconds have not elapsed, picking of documents is retried at a shorter interval (in units of 0.5 second in this example) at step S8. On the other hand, if it is determined, at step S7, that the 2 seconds have elapsed, judging that the document has not been conveyed normally, pick error processing is performed (S9). Then, the conveyance motor is stopped (S10), and the operator confirms the existance of the document again, and repeats the pick operation.
On the other hand, if, at step S6, it is determined that a document has been detected by SF1, and judging that some trouble has occurred because no document has been detected by any of size sensors at step S5, pick error processing is performed at step S9. Then, at step S10, the conveyance motor is stopped.
If, at step S5, it is determined that any of the size sensors has detected the document, and judging that picking of the document has been normally completed, the name of the size sensor that was first turned on is stored in a memory (S11), and counting of the pulses of the conveyance motor after the size sensor detected the document is started (S12). Next, the present state of the sensor is stored in the memory (S13). At step S14, it is determined whether a size sensor for a size exceeding the set document size is ON or OFF. If it is determined to be ON, the process moves to step S15. If it is determined to be OFF, the process moves to step S17.
At step S15, the skew error correction processing is performed. That is, at step S16, the conveyance motor is stopped, and then the pick operation is repeated. At step S17, it is determined whether or not SF1 has detected the document. If, at step S17, it is determined that SF1 has not detected the document, it is checked, at step S18, whether or not the count of pulses has exceeded a specified number of pulses. Since necessary number of pulses after the size sensor detects the document until SF1 detects the same document, when the document is conveyed normally, is known beforehand, it can be determined whether the document has been conveyed normally or not by counting the pulses after the size sensor detects the document until SF1 detects the same document and by comparing the number of counted pulses with the specified number.
If the counted number exceeds the specified number, and judging that the document has not been conveyed, jam error processing is performed (S19). That is, the conveyance motor is stopped at step S20, and then the pick operation is repeated. On the other hand, if, at step S18, the counted number does not exceed the specified number of pulses, the process returns to step S13.
On the other hand, at step S17, if SF1 has detected the document, the first clutch is turned off and driving of the pick roller is stopped (S21). Then, calculation of skew (skew angle) is performed (S22). At step S23, it is determined whether or not an abnormal deviation (skew angle of 10xc2x0 or more) has occurred. If it is determined that abnormal deviation has occurred, the process moves to step S15. If it is determined that abnormal deviation has not occurred, the process moves to step S24. Then, the pulses of the conveyance motor after SF1 detected the document are counted (S24). Thereafter, it is determined whether the surface reading timing sensor SF2 has detected the document or not (S25).
If, at step S25, it is determined that SF2 has detected the document, the second clutch is turned off and pick operation for the next document is performed at S26. On the other hand, if, at step S25, it is determined that SF2 has not detected the document, it is determined at step S27 whether or not the counted number of pulses has exceeded the specified number of pulses. If the counted number has exceeded the specified number of pulses, jam error processing is performed (S19). If the counted number has not exceeded the specified number of pulses, the process returns to step S25, where it is again determined whether or not SF2 has detected the document.
Here, in the case of an image reading apparatus, the skew of the conveyed medium manifests itself as tilting of the read image. Therefore, the occurrence of skew error has been conventionally detected based on the degree of tilting of read image.
FIG. 6 is a view useful for explaining the conventional method of skew detection. In this figure, the portion enclosed by dotted line shows the state of a document read when no skew has occurred, as seen on a display. The portion enclosed by solid line shows the state of the document read when skew has occurred, as seen on a display.
In the figure, A shows the amount of skew in the main scanning direction, and corresponds to the difference between left end and right end of the leading edge of the document. B shows the amount of skew in the subscanning direction, and, in this figure, corresponds to the difference of the leading edge and the trailing edge of the document.
In this detection method, the proportion of the values A and B shown in FIG. 6, in relation to the length of sheet in the main scanning direction and in the subscanning direction respectively, was determined, and if A/B was 1% or less of the length of sheet in the main-/sub-scanning direction, respectively, the document was regarded as normal, and if A/B was beyond this range, the document was determined to be abnormal.
However, with this detection method, an operator had to identify the difference in both main- and sub-scanning directions on the display. Thus, for each document read, an operator had to identify on the display whether skew error has occurred or not, and this put a heavy burden on the operator.
Further, in recent years, the operation speed of a document reading apparatus has become increasingly higher so that it has become more and more difficult for an operator to confirm whether skew error has occurred or not for each document. Even if occurrence of skew error is identified, there is another problem that it is very difficult to perform proper skew error correction processing corresponding to the skew error rapidly.
When amount of skew error is calculated from the read image, the document usually has a white background, and the images or the characters are colored in most cases. Thus, in order to detect the skew of the document reliably, a greater contrast at the time of reading is more desirable. Therefore, it is desirable that color of the backing of the image reading unit be set as black so as to allow more reliable judgement of the skew of the medium. However, in order to read an image with black backing and to determine the skew angle reliably, processing such as the detection of the edge portion of the backing and the document from the read image is necessary so that calculation of the amount of skew becomes very time consuming.
In reading an ordinary image, in order to reproduce the gradation of the document, it is also necessary to read the output value of white color in the reader unit and to thereby correct the reference value. This is because it is difficult, in the reader unit, to illuminate the document, by the light source, uniformly in the main scanning direction, with light intensity generally decreasing toward both ends of a light source if the light source is a long device such as a fluorescent lamp. There is also a fluctuation in the sensitivity of a CCD that converts the reflected light into an electrical signal so that a non-uniform signal is generated from the CCD each time of reading a document, leading to another problem that the reproduction of accurate gradation of the document is not possible. In addition, as the temperature of the light source rises as time passes, the output voltage becomes higher even if the same color is read. As a result, when the read image is output and compared with the actual document, it sometimes happens that a grey document turns out to be a white image. Therefore, it is necessary to reproduce the gradation of the actual document accurately by reading the output voltage of white for reference before the image is read by the image reader, and by correcting the output signal from the CCD using the reference value. Especially in the case of reading of color images, that has become popular recently, correction of the white reference value is essential.
Thus, it is desirable to use a black backing for reading an image or detecting the amount of skew. However, in order to follow the variation of light intensity or the fluctuation of the CCD accurately, the reference value is preferably corrected using a white backing before reading of image. In an actual image reading apparatus, the color of the backing is, in most cases, fixed to be either black or white. If the color of the backing is switched each time a document is read, a switching time is required and various processings become very time consuming, so that a high speed apparatus cannot be realized.
Other means for detecting the amount of skew include a method using sensors for detecting the size or the passage of a medium. The amount of skew is detected from the difference of passage time between sensors. An apparatus having means for automatic correction for the skew of a medium based on the detected amount of skew, that is, an apparatus having a function for performing skew error correction by rotating the read image based on the detected amount of skew, is now widely used.
However, in an image reading apparatus which conveys a stacked medium along conveyance path, various kinds of sheets are conveyed including different paper qualities and different kinds of sheets are used. Thus, even if documents are set in the image reading apparatus so as to properly adjust the size of documents to the guide of the apparatus, deviation of the center of the documents from the center of the apparatus may sometimes occur in the course of actual conveyance. This occurs because the width of the sheet guide is set a little greater than the actual width of the sheets. When documents are conveyed with the center deviating from the center of the apparatus, it becomes impossible to convey the documents with the conveyance roller kept at the centerline of the documents. This leads to a skew of the document before the document reaches the image reader.
When the document is smaller in size than the width of the guide, the document may sometimes reach the image reader in a rotating state.
In the case of an image reading apparatus with a high-speed processing function, the amount of skew detected by the sheet passage sensor may sometimes differ from the actual amount of skew when the document arrives at the image reader. This is because the amount of skew continues to increase while the medium is being conveyed. In this case, in order to decide the degree of skew error correction to be performed on the read image, it is important to define the time of skew detection to be used as the basis for performing the correction.
There has been another problem in conventional skew error correction in that a predetermined skew error correction is performed uniformly irrespective of the amount of skew and that skew error correction is performed even in the case where there is no need for correction so that the correction becomes very time consuming.
It is an object of the present invention to perform skew error correction processing as required by the actual amount of skew at high speed without using unnecessary time.
It is another object of the present invention to realize an accurate and high-speed image reading process that is capable of correcting the white reference value at the time of image reading by switching the backing of the image reader automatically between black and white as necessary, and of detecting and correcting an accurate amount of skew in the image reader when there is a large amount of skew.
To resolve the above problems, according to the present invention, in an image reading apparatus which conveys a stacked medium along a conveyance path, reads an image of the medium during conveyance, and discharges the medium after reading the image, said image reading apparatus is comprised of calculation means for calculating the amount of skew of said medium during conveyance, and correction means for correcting the amount of skew of said medium in accordance with the amount of skew calculated by said calculation means.
In the above image reading apparatus, said correction means include determination means for determining whether correction for said amount of skew should be performed or not, and control means for correcting the amount of skew of said medium when said determination means determine that correction for said amount of skew should be performed.
In the above image reading apparatus, said calculation means include medium detection means for detecting the state of medium being conveyed along said conveyance path, wherein said amount of skew is calculated from the detection result of the state of the medium obtained by the medium detection means, and based on the amount of skew of said medium calculated by said calculation means, said determination means determine whether correction for said amount of skew should be performed or not.
In the above image reading apparatus, said control means include first correction control means for correcting the amount of skew based on said amount of skew of the medium calculated by said calculation means when said determination means determine that correction for said amount of skew should be performed.
In the above image reading apparatus, said calculation means include image reading means for reading the image on the medium at a position on the conveyance path closer to the discharging side than said medium detection means for the medium conveyed along said conveyance path, said amount of skew being calculated from the image data read by the image reading means, and based on the amount of skew of said medium calculated by said calculation means, said determination means determine whether correction for said amount of skew should be performed or not.
In the above image reading apparatus, said control means include second correction control means for correcting the amount of skew based on said amount of skew of the medium calculated by said calculation means when said determination means determine that correction for said amount of skew should be performed.
In the above image reading apparatus, said calculation means comprise image reading means for reading an image of the medium being conveyed along said conveyance path at a point closer on the conveyance path to the discharging side than said medium detection means, and calculate said amount of skew from the image data of the medium read by the image reading means, and based on the amount of skew of said medium calculated by said calculation means, said determination means determine whether correction for said amount of skew should be performed or not.
In the above image reading apparatus, said control means include selection means for selecting, when said determination means determine that correction for said amount of skew should be performed, so as to perform correction for the amount of skew with a first correction control means that amends the amount of skew based on an amount of skew of the medium calculated from the detected result of the state of the medium by the medium detection means if the amount of skew is within a predetermined amount, and with a second correction control means that amends the amount of skew based on an amount of skew of the medium calculated from the image data of the medium read by the image reading means if the amount of skew exceeds the predetermined amount.
In the above image reading apparatus, said second correction control means include a plurality of backings which are provided at a position opposite to said image reading means for reading an image of said medium being conveyed in relation to said medium, and which change the reflectivity of light incident on the medium from the side of the image reading means, and backing switching means for selectively switching to one of said plurality of backings, wherein, when said determination means determine that correction for said amount of skew should be performed, the backing is switched to the one selected by said backing switching means, image data on said medium being read after the switching, said amount of skew being calculated from the read image data by the calculation means, and said amount of skew being corrected based on the calculated result.
In the above image reading apparatus, calculation of said amount of skew is performed such that the position of the corner of said medium is first read from said image data, and said amount of skew is calculated from the position data of said corner and conveyance speed of said medium conveyed on said conveyance path.
To resolve the above problems, in an image reading method which comprises the steps of conveying stacked medium along a conveyance path, reading an image on the medium being conveyed, and discharging the medium after reading the image, said image reading method according to the present invention further comprises a calculation step of calculating the amount of skew of said medium during conveyance, and a correction step of correcting said amount of skew of said medium in accordance with the amount of skew calculated by said calculation means.
In the above image reading method, said correction step comprises a determination step of determining whether said correction for the amount of skew should be performed or not, and a control step of correcting the amount of skew of said medium when the determination step determines that said correction for the amount of skew should be performed.
In the above image reading method, said calculation step comprises a medium detection step of detecting the state of medium being conveyed along said conveyance path, wherein said amount of skew is calculated from the detection result of the state of the medium obtained in the medium detection step, and said determination step determines whether correction for said amount of skew should be performed or not based on the amount of skew of said medium calculated in said calculation step.
In the above image reading method, said control step comprises a first correction control step of correcting the amount of skew based on said amount of skew of the medium calculated in said calculation step when said determination step determines that correction for said amount of skew should be performed.
In the above image reading method, said calculation step comprises an image reading step of reading the image on the medium at a position on the conveyance path closer to the discharging side than the position of the medium conveyed along said conveyance path at the time of detection, said amount of skew being calculated from the image data read, and said determination step determines whether correction for said amount of skew should be performed or not based on the amount of skew of said medium calculated in said calculation step.
In the above image reading method, said control step comprises a second correction control step of correcting the amount of skew based on said amount of skew of the medium calculated in said calculation step when said determination step determines that correction for said amount of skew should be performed.
In the above image reading method, said calculation step comprises an image reading step of reading an image of the medium at a position closer on the conveyance path to the discharging side than the position at the time of detection of said medium being conveyed along said conveyance path, and calculates said amount of skew from the image data of the medium read by the image reading step, and said determination step determines whether correction for said amount of skew should be performed or not based on the amount of skew of said medium calculated in said calculation step.
In the above image reading method, said control step comprises a selection step of selecting, when said determination step determines that correction for said amount of skew should be performed, so as to perform correction for the amount of skew by the first correction control step for amending the amount of skew based on an amount of skew of the medium calculated from the detected result of the state of the medium in the medium detection step if the amount of skew is within a predetermined amount, and by a second correction control step for amending the amount of skew based on an amount of skew of the medium calculated from the image data of the medium read in the image reading step if the amount of skew exceeds the predetermined amount.
In the above image reading method, said second correction control step comprises a backing switch step of selecting and switching to one of a plurality of backings which are provided at a position opposite to an image reading means for reading image on said medium being conveyed in relation to said medium, and which change the reflectivity of light incident on the medium from the side of the image reading means, wherein, when said determination step determines that correction for said amount of skew should be performed, the backing is switched to the one selected in said backing switch step, image data on said medium being read after the switching, said amount of skew being calculated from the read image data in the calculation step, and said amount of skew being corrected based on the calculated result.
In the above image reading method, calculation of said amount of skew is performed such that the position of the corner of said medium is first read from said image data, and said amount of skew is calculated from the position data of said corner and conveyance speed of said medium conveyed on said conveyance path.